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The Graphics Pipeline CS2150 Anthony Jones Introduction • What is this lecture about? – The graphics pipeline as a whole – With examples from the video games industry • Definition – The sequence of steps that are applied to a graphics primitive before it may be visually represented. – The graphics pipeline typically accepts some representation of a three-dimensional scene as an input and results in a 2D raster image as output.* Introduction Introduction Introduction • Games are significant drivers for current advances in computer graphics technology • The global games market is worth £18bn and growing at 9% per annum* • The video gaming industry is estimated to be worth £500m to the UK economy** • Games made in the UK between 2006 and 2008 alone are on track to generate global revenues of £4bn* Introduction Transistors Millions • Games are significant drivers for current advances in computer graphics technology • Increasing power of dedicated rendering hardware 1600 1400 1400 1200 1000 800 754 754 600 400 302 200 222 1 22 63 130 0 1995 1999 2002 2003 2004 2005 2006 2007 2008 Introduction • Games are significant drivers for current advances in computer graphics technology • Increasing power of dedicated rendering hardware Core Clock Max (MHz) 600 500 400 300 200 100 0 1997 1998 1999 2000 2001 2002 2003 2004 2005 Video • Examples of pre-dedicated hardware games • Proprietary software renderers – Descent – Quake Fixed Function Graphics Pipeline Application Transform & Lighting Rasterisation In: • 3D scene information • Geometry and attributes Fragment Processing Frame Buffer Out: • 2D raster image • Pixel position and colour Fixed Function Graphics Pipeline Application Transform & Lighting • Conveyor belt analogy – Processing stages – All stages benefit from parallel processing Rasterisation Fragment Processing Frame Buffer Fixed Function Graphics Pipeline Application Transform & Lighting Rasterisation Fragment Processing Frame Buffer • Disclaimer – The position of certain operations in the pipeline (e.g. clipping and culling): • May not be consistent with what you may have read or heard elsewhere • Can take place at multiple stages in the pipeline • May depend on graphics hardware vendor (e.g. nVidia vs. ATI), graphics card family and API (e.g. DirectX vs. OpenGL) • Will change over time as graphics pipelines evolve Application Transform & Lighting Rasterisation • Geometry submission – Typically vertices, normals and object-space texture coordinates • Attribute submission – Material and colour settings, lights, texture bindings • In OpenGL: – glBegin, glEnd, glVertex and glNormal – OpenGL state machine manipulations Fragment Processing Frame Buffer • Object/World space Application Transform & Lighting Rasterisation • ModelView* transform View space – Normalize normals – Vertex lighting – Generate eye-space texture coordinates • Trivial rejection and back face culling Fragment Processing Frame Buffer • Object/World space Application Transform & Lighting Rasterisation • ModelView* transform View space – Normalize normals – Vertex lighting – Generate eye-space texture coordinates • Trivial rejection and back face culling Fragment Processing • View space Frame Buffer Application Transform & Lighting Rasterisation • Projection* transform and perspective (w) divide Clip space – Normalised Device Coordinate cube • x and y from -1 to 1 • z from 0 to 1 – Gives depth cues such as perspective foreshortening and motion parallax • Clipping – Clipped geometry is retesselated Fragment Processing • View space Frame Buffer Application Transform & Lighting Rasterisation • Projection* transform and perspective (w) divide Clip space – Normalised Device Coordinate cube • x and y from -1 to 1 • z from 0 to 1 – Gives depth cues such as perspective foreshortening and motion parallax • Clipping – Clipped geometry is retesselated Fragment Processing • Clip space Frame Buffer Application Transform & Lighting Rasterisation • Projection* transform and perspective (w) divide Clip space – Normalised Device Coordinate cube • x and y from -1 to 1 • z from 0 to 1 – Gives depth cues such as perspective foreshortening and motion parallax • Clipping – Clipped geometry is retesselated Fragment Processing • Clip space Frame Buffer Application Transform & Lighting Rasterisation • Projection* transform and perspective (w) divide Clip space – Normalised Device Coordinate cube • x and y from -1 to 1 • z from 0 to 1 – Gives depth cues such as perspective foreshortening and motion parallax • Clipping – Clipped geometry is retesselated Fragment Processing • Clip space Frame Buffer Application Transform & Lighting Rasterisation • Projection* transform and perspective (w) divide Clip space – Normalised Device Coordinate cube • x and y from -1 to 1 • z from 0 to 1 – Gives depth cues such as perspective foreshortening and motion parallax • Clipping – Clipped geometry is retesselated Fragment Processing • Clip space Frame Buffer Application Transform & Lighting Rasterisation • Scale and translate Screen space – The current viewport maps clip space to the frame buffer – 3D vertices are finally transformed to a 2D coordinate system – Although z and w are retained for fragment processing Fragment Processing • Clip space Frame Buffer Application Transform & Lighting Rasterisation • Scale and translate Screen space – The current viewport maps clip space to the frame buffer – 3D vertices are finally transformed to a 2D coordinate system – Although z and w are retained for fragment processing Fragment Processing • Screen space Frame Buffer Application Transform & Lighting Rasterisation • Maps continuous primitives to the frame buffer’s discrete grid • Interpolates colour, texture coordinates and depth values across fragments • Converts primitives into fragments (not pixels!) • Scan conversion Fragment Processing • Screen space Frame Buffer Application Transform & Lighting Rasterisation • Fragments – Transient representations – Frame buffer position (x/y coordinates), colour and depth • Fragments are not pixels! – Pixels belong to the frame buffer – Fragments could be considered potential pixels Fragment Processing • Screen space Frame Buffer Application Transform & Lighting Rasterisation • Fragment operations* – Fragments may be rejected by tests such as scissor, stencil, alpha and depth (z-buffer) • Fragment shading based on: – Vertex colour attributes – Shading system in use (e.g. flat, Gouraud, Phong) – Texture lookup • Special effects – Including fog, environment mapping, bump mapping, and shadows Fragment Processing • Screen space Frame Buffer Application Transform & Lighting Rasterisation • Pixel memory on the graphics card – May be output to the screen, or retained for further use (buffered) • Texture targets – Available to the application for special effects, scene feedback (occlusion culling, shadow mapping), etc. Fragment Processing • Screen space Frame Buffer Video • Examples of fixed functionality games – Quake 2 – Half life Programmable Graphics Pipeline Application Transform & Lighting • Vertex programs replace fixed transform and lighting • In: 1 vertex – – – – 3D position Normal Texture coords Colour • Out: 1 vertex – – – – – 3D position Normal Texture coords Colour Screen space position Rasterisation Fragment Processing Frame Buffer Programmable Graphics Pipeline Application Transform & Lighting Rasterisation • Fragment programs replace fixed fragment processing and selection • In: 1 fragment – – – – – – 3D position Normal Texture coords Colour Screen space position Depth • Out: 0 or 1 fragment – Colour – Depth Fragment Processing Frame Buffer Video • Examples of games based on today’s programmable technology – Quake 3 Arena – Doom 3 – Gears of War – Ghost Recon Advanced Warfighter – FarCry 2 Modern Graphics Pipeline Application Transform & Lighting Primitive Assembly Rasterisation Memory Resources (e.g. Buffers and Textures) • Geometry programs allow primitive assembly and manipulation prior to rasterisation • New memory resources with more flexible read/write access Fragment Processing Frame Buffer Video • Examples of what will be available soon – nVidia’s Human head demo – WarDevil – Project Offset – Gran Turismo 5 – Smoke and Water demos Summary Application Transform & Lighting Rasterisation Fragment Processing Frame Buffer • The overall graphics pipeline is still here... Summary Application Transform & Lighting Primitive Assembly Rasterisation Fragment Processing Memory Resources (e.g. Buffers and Textures) • Although a few improvements have been made: – More programmable and flexible • Video games continue to push the envelope, with: – More content and detail • Video games are starting to look like computer generated films... Frame Buffer
